Multilayer Seed Stack for Smooth MTJ Films and Stable PMA
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Solution Overview
Problem
Current MTJ structures fail to maintain perpendicular magnetic anisotropy (PMA) and thermal stability of the free and reference layers during high temperature semiconductor processing up to 400°C, which is essential for advanced embedded MRAM devices.
Innovation Solution
A multilayer seed layer stack comprising a high resputtering rate layer and a low resputtering rate amorphous layer is used to create a smooth top surface, promoting PMA in the overlying magnetic layers, which includes a template layer with a (111) crystal orientation and materials like NiW, NiMo, NiCr, or Hf/NiFeCr, combined with a laminated reference layer and tunnel barrier to enhance thermal stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional single-layer seed layer is used, then the device structure is simple and easy to manufacture, but the perpendicular magnetic anisotropy (PMA) and thermal stability are lost during high temperature processing up to 400°C
Solution Approach 1:
The seed layer is divided into multiple sub-layers with different functions: a first seed layer (e.g., Ta, Ru, or Mo) providing adhesion and a second seed layer (e.g., NiCr, NiFeCr, or CoFeB) providing PMA promotion. This segmentation allows each layer to be optimized for its specific function, enabling thermal stability during 400°C processing while maintaining a manageable structure.
Solution Approach 2:
The invention uses composite seed layer structures combining different materials with complementary properties. The first seed layer material is selected for thermal stability and adhesion, while the second seed layer material is selected for promoting PMA. This composite approach maintains PMA and thermal stability through high temperature processing that would degrade conventional single-layer seeds.
2Reliability
If the seed layer is optimized for PMA promotion, then perpendicular magnetic anisotropy is maintained, but the resputtering rate becomes too high causing excessive surface roughness
Solution Approach 1:
The invention applies local quality by giving different parts of the seed layer structure different properties: the first seed layer has low resputtering rate for smooth surface formation, while the second seed layer has high resputtering rate for PMA promotion. This local differentiation allows the bottom interface to remain smooth while the top interface promotes PMA, resolving the contradiction between surface quality and magnetic property enhancement.
Solution Approach 2:
The first seed layer acts as an intermediary between the substrate and the second seed layer. It provides a smooth, stable foundation with low resputtering that protects the underlying structure, while allowing the second seed layer to perform PMA promotion. This intermediary layer mediates between the conflicting requirements of surface smoothness and PMA enhancement.
3Manufacturing precision
If a smooth seed layer surface is used, then surface roughness is low improving film quality, but PMA promotion capability is reduced
Solution Approach 1:
The invention resolves the contradiction by moving from a single-dimensional optimization (either smooth or PMA-promoting) to a multi-dimensional solution with stacked layers. The first layer optimizes for surface smoothness in one dimension, while the second layer optimizes for PMA promotion in another dimension. This dimensional expansion allows both requirements to be satisfied simultaneously in different parts of the structure.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The multilayer seed layer stack maintains PMA and enhances thermal stability of the magnetic layers, ensuring reliable performance of MRAM devices even after high temperature processing.
Implementation Method 1
A multilayer stack is selected for the seed layer wherein the seed layer comprises a stack of layers including a bottommost layer, a second layer contacting a top surface of the bottommost layer and having a higher resputtering rate than the bottommost layer, and a third layer contacting a top surface of the second layer and having a lower resputtering rate than the second layer
Implementation Method 2
The tunnel barrier is typically about 10 Angstroms thick so that a current through the tunnel barrier can be established by a quantum mechanical tunneling of conduction electrons
Implementation Method 3
MTJ elements wherein one or both of the free layer and reference layer have perpendicular magnetic anisotropy (PMA)
Data Source
AI summary
A method of forming a seed layer stack for a magnetic device may include depositing a bottom seed layer, forming at least one pair of smoothing layers over the bottom seed layer, and depositing a top seed layer over and abutting the at least one pair of smoothing layers. Forming at least one pair of smoothing layers may include sputter depositing a sub-smoothing layer over the bottom seed layer and sputter depositing an amorphous sub-smoothing layer over and abutting the sub-smoothing layer. A top surface of as-sputter deposited sub-smoothing layer has a first top surface roughness, and the sputter depositing of the amorphous sub-smoothing layer causes re-sputtering of the sub-smoothing layer, such that the top surface of the as-sputter deposited sub-smoothing layer has a second top surface roughness less than the first top surface roughness. The sub-smoothing layer, the bottom seed layer, and the top seed layer include different materials.